1. Field of the Invention
The present invention relates to a receiving apparatus and a method of outputting a received signal in multimedia communication, more particularly, to a receiving apparatus and a method of outputting a received signal in multimedia communication in which a transmitting apparatus multiplexes plural kinds of information such as a video signal, an audio signal, etc. and transmits the multiplexed information through a communication line, and in which a receiving apparatus separates the multiplexed information and outputs the separated information to predetermined external terminals.
2. Description of the Related Art
FIG. 11 schematically shows the principle of a multimedia broadcast communication system using a satellite transponder. In FIG. 11, the reference numeral 1 represents a transmitting station, 2.sub.1, 2.sub.2, . . . receiving stations and 3 a satellite transponder. In the transmitting station 1, the reference numeral 1a represents a video transmitter, 1b an audio transmitter, 1c a time division multiplexer, 1d a transmitter and 1e a transmission antenna. The video transmitter 1a inputs digital video data with a forward error correction code (FEC) attached thereto to the time division multiplexer 1c, and the audio transmitter 1b inputs digital audio data with an FEC attached thereto to the time division multiplexer 1c. The time division multiplexer 1c multiplexes the data such as video data and audio data output from plural kinds of media, and the transmitter 1d adds a preamble PA including carrier and clock recovery sequence (CCA), unique word (UW), control information (CI), and etc. to the multiplexed data DT, as shown in FIG. 12. The transmitter 1d then modulates the multiplexed data by, for example, quadrature phase shift keying (QPSK) and transmits it from the transmission antenna 1e at a predetermined time slot.
In the receiving station, the reference numeral 2.sub.a represents a reception antenna, 2b a demodulator, 2c a demultiplexer, 4 an image receptor such a TV and 5 an audio unit.
The demodulator 2b demodulates the signal received by the antenna 2a into the data. The demultiplexer 2c separates the received data into the data for each information medium and inputs the video data to the image receptor 4 and the audio data to the audio unit 5.
In FIG. 11, only one transmitting station is shown, but a channel of the same satellite transponder 3 is generally used by a multiplicity of transmitting stations by using time-sharing technique and the data from the desired transmitting station is extracted by the corresponding receiving station. In this case, the transmitter 1d at each transmitting station adds a preamble to the multiplexed data after the compression of the time axis thereof, then performs phase modulation and transmits the modulated signal from the transmission antenna 1e at the determined time slot within a TDMA frame. Thereafter, each transmitting station intermittently transmits the signal by using a carrier of a burst mode at every predetermined interval of time. In this way, a multiplicity of transmitting stations use the same satellite transponder 3 by using time-sharing technique.
In the above-described multimedia broadcast communication system, if the communication network quality is deteriorated (for example, in the case of a satellite link, if the C/N is degenerated due to rainfall), the image on the image receptor 4 is disturbed, or it is difficult, or sometimes impossible to hear the audio signal output from the audio unit 5. In such a case, some output control by the receiving apparatus at the receiving station is required. For example, the receiving apparatus stops outputting a video signal and an audio signal to the terminals (e.g., the image receptor and the audio unit), or holds the state of the preceding screen so as to display it, or displays a fixed screen.
FIG. 13 shows the structure of a receiving apparatus which explains the conventional output control executed when the communication network quality is deteriorated. The same numerals are provided for the elements which are the same as those shown in FIG. 11.
The reference numeral 2 represents a receiving apparatus, 2a a reception antenna, 2b a demodulator for demodulating data from a received signal by extracting a reference carrier which is necessary for synchronous detection and a clock which is necessary for the reproduction of a code by using the CCR (carrier and clock recovery sequence), 2c a demultiplexer for separating the received data into the data for each medium, and 2d an output controller. The output controller 2d outputs video data, audio data and other media data to the corresponding terminals, and stops outputting data when the communication network quality is deteriorated. The reference numeral 2e represents a UW detector for detecting a unique word UW (having a bit pattern of a predetermined length) from demodulated data, 2f a synchronizer for outputting a pulse P.sub.uw (see FIG. 14) for defining the head of the unique word UW and, when no unique words UW is detected, inputting a data stop signal DSP to the output controller 2d, and 2g a timing signal producer for outputting timing signals VDS, ADS (see FIG. 14) which attain high levels during the data acquisition periods of the respective media.
When the communication line quality is good, the unique word UW is detected by the UW detector 2e, and the timing signal producer 2g outputs the timing signals VDS, ADS to the respective media. The demultiplexer 2c outputs demodulated video data to a line L.sub.v when the timing signal VDS has a high level, and the output controller 2d inputs the video data to the image receptor 4. The demultiplexer 2c outputs demodulated audio data to a line L.sub.a when the timing signal ADS has a high level, and the output controller 2d inputs the audio data to the audio unit 5. Similarly, other media data are input to the corresponding terminals.
On the other hand, if the communication network has some trouble and cannot detect the unique word UW and, hence, synchronous reception is impossible, the synchronizer 2f inputs the data stop signal DSP to the output controller 2d. The output controller 2d then stops outputting video data, audio data, etc. to the corresponding terminals.
In this system the data stop signal DSP is not supplied unless the communication line quality is considerably bad. However, signals on some media are often disturbed or become difficult to hear even if the communication line quality is not so bad as to supply the data stop signal DSP. As a result, in the conventional received signal output control system, even if signals are disturbed or become difficult to hear, they are not stopped outputting, and it is not until the communication line quality is deteriorated so much as to make it impossible to display an image on the screen or to hear sound that the signals are stopped outputting. It cannot therefore be said that the output control effectively functions in this system. That is, in the conventional system for stopping outputting signals only on the basis of the result of UW detection, there is a problem that when the communication line quality is deteriorated to a certain degree, it is impossible to use some media.